The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the measurement and limitation of RF power produced by an MRI system during a patient scan.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a radio frequency (RF) magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1. is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which an RF excitation pulse is applied and these gradients are varied according to a particular localization method. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. Such pulse sequences may also employ RF refocusing pulses, RF saturation pulses and other types of RF pulses required by the prescribed scan.
Very high field MR systems (such as MR scanners operating at a main field strength of 3.0 Tesla (T)) are becoming more widely available. An enabling technology is the compact, actively shielded magnets, which recently became available. This technology permits the 3.0 T MRI system to be sited in a clinical setting. Clinical applications including pulse sequences, and parameter selections (i.e. protocols) are being developed especially for these high field scanners.
A major limitation of scanning at very high field is the radiofrequency (RF) power deposited in the patient, as measured by the specific absorption rate (SAR). SAR increases approximately quadratically in the range of 1.5 T to 3.0 T. Therefore, applications which are straightforward to implement at standard fields strengths such as 1.5 T can be severely limited by SAR at higher field strengths such as 3.0 T. Specific guidelines for the maximal amount of SAR that may be deposited in the patient are specified by the Food and Drug Administration (FDA) in the United States, and by other regulatory agencies in other countries. If SAR limits are exceeded, undesirable and possible dangerous patient heating may result.
To ensure that SAR deposition is within acceptable limits, prior MR systems employ a number of measures. In one method, the RF power deposited by a particular pulse sequence is estimated with a calculation based on the shape, amplitude, and duration of each of the RF pulses within the pulse sequence. If the estimated SAR for a given pulse sequence exceeds regulatory limits, then the software automatically limits input parameters such as the maximal number of slices, flip angle, or minimal repetition time (TR).
Another method used in commercial MR systems employs power monitor hardware and software. The power monitor measures power transmitted by the RF coil in the MR system. In one commercial system, the average RF power delivered by the RF coil is measured at regular time intervals, approximately every 30 milliseconds (ms). A moving average of approximately 33 consecutive power measurements is calculated. Thus, the averaging time for this system is 30 msxc3x9733 measurements, which is approximately 1 second. If at any time this moving average of measured power exceeds a predetermined limit (e.g. 10 Watts for head coil studies), the power monitor xe2x80x9ctripsxe2x80x9d, and the scan is aborted.
A major limitation of the prior methods is many MR pulse sequences contain periods over which there is relatively intense application of RF pulses, followed by relatively quiescent or xe2x80x9cdeadxe2x80x9d periods. In this case, the 1-second average time can be overly restrictive since it can cause power monitor trips that are not necessary to protect the patient from detrimental heating. For example, if the power monitor trip point is set to 10 Watts, and during a 1-second period of intense RF pulse activity the average RF power is 15 Watts, the power monitor would trip. The scan is thus aborted, even if the active period is followed by a 9-second dead time. In the 1-second active period, 15 Joules of energy is deposited into the patient, (assuming 100% coil coupling efficiency). This 15 Joules is an insufficient amount of energy to cause detrimental patient heating. Over the 1+9=10 second interval, the average power is only 15 J/10 s=1.5 Watts, which is well within safe limits. Thus, the scan is unnecessarily aborted.
Simply increasing the averaging period to a value above 1-second is not a safe solution to this problem. For example, if the averaging time is increased to 333 samples, or approximately 10 seconds and 100 Watts is delivered continuously to the patient, then up to 1000 Joules of energy may be deposited before the scan is aborted. This amount of energy could cause harm to the patient.
The present invention is a method and apparatus for monitoring the RF power applied to a patient during a scan, and altering the scan when excessive RF power is detected. The RF power produced during a scan is measured and a plurality of moving averages of this measured power over a corresponding plurality of different accumulation time intervals are calculated. Associated with each accumulation time interval is a different RF power trip level. If any moving average exceeds the RF power trip level for its accumulation time interval the scan is aborted or altered.
A general object of the invention is to protect, the patient from harmful heating, and avoid unnecessary power monitor trips. If it is determined that 50 Joules of energy is the maximum safe limit to be deposited within the patient in a 1 second period, then the first accumulation time is 1 second, and the RF power trip level is 50 Watts. If it is further determined that within a 5 second period it is safe to deposit 75 Joules, then the trip level for the 5 second accumulation time interval is set to 15 Watts. The longest accumulation time interval is set to 10-30 seconds, which is on the order of the longest repetition time encountered in MR pulse sequences. The trip level for the longest time interval is set to the lowest trip level, for example, 10 Watts. The trip level for this longest accumulation time interval corresponds with the regulator limits set by the FDA.